Illinois, the home state of Barack Obama, who won state with a 25.1% margin of victory. Prior to the election, all 17 news organizations considered this a state Obama would win, or otherwise considered as a safe blue state. One of the most reliably blue states in the nation, Illinois has not voted for a Republican presidential nominee since 1988, when George H.W. Bush narrowly carried the state. In 2008, continuing that trend, it appeared that a generic Democratic presidential nominee could have easily won Illinois. Thus, it surprised no one that Barack Obama, who represented Illinois in the U.S. Senate, won Illinois in 2008 over Republican John McCain in a landslide victory, clinching near 62% of the total vote.

It was no surprise that Barack Obama cruised to a landslide victory in Illinois, the state he had represented in the U.S. Senate since 2004. He enjoyed massive support in his state among all demographics. According to exit polls, 57 percent of voters in the Illinois Democratic Primary were Caucasian and they opted for Obama 57-41; 24 percent of voters were African American and they, too, backed Obama 93-5; and 17 percent of voters in the primary were Hispanic/Latino and they narrowly backed Obama 50-49. Obama won all age groups but tied Clinton among senior citizens aged 65 and over. He won all voters in the state of all educational attainment levels as well as income/socioeconomic classes. He won all ideological groups and voters from both parties as well as self-identified Independents. Regarding religion, Obama won every major denomination except Roman Catholics, who narrowly backed Clinton 50-48 percent. Obama won Protestants by a margin of 58-38, other Christians 79-19, other religions 82-17, and atheists/agnostics 78-21.

Results of the Illinois Democratic Primary by County. Dark blue counties were won by Obama; gray counties were won by Clinton.

Obama performed extremely well statewide and wracked up massive victories in his home city of Chicago as well as its suburbs and the metropolitan area. He also won Northern Illinois as well as the collar counties by substantial victories. Clinton’s best performance was in Southern Illinois among the more rural and conservative counties that are majority white, although Obama still won the region as a whole.

The Illinois Republican primary, 2008 was held on February 5, 2008. Illinois was one of 24 States holding a primary or caucus on Super Tuesday. Delegates from each of Illinois' 19 congressional districts are selected by direct election. In addition, the primary ballot also contains a preference poll that lists the presidential candidates.

For most of the second half of the 20th century, Illinois was reckoned as a Republican-leaning swing state. It voted Republican in every election from 1952 to 1988, save for 1960 and 1964. However, George H. W. Bush just barely won the state in 1988, and it swung heavily to Bill Clinton and the Democrats in 1992. Since then, Democrats have won the state by fairly comfortable margins, and it is now reckoned as the most solidly Democratic state in the Midwest.

The blue trend in the Land of Lincoln in presidential elections can be largely attributed to Cook County, home to Chicago, which makes up about 41.2% of the state's total population.[23] While Chicago has been a Democratic stronghold for decades, the suburban areas of Cook County have historically tilted Republican. The brand of Republicanism in the suburbs, however, has historically been a moderate one, and these areas swung Democratic as the national party moved more to the right. Democrats also do very well in the Illinois portions of the Quad Cities and St. Louis areas. Additionally, the historically Republican collar counties near Chicago--DuPage, Lake, McHenry, Kane, and Will--have become friendlier to Democrats at the national level.

Barack Obama, the junior U.S. Senator from Illinois at the time of the election, carried the state handily, defeating John McCain of Arizona by a margin of 1.38 million votes. Obama carried his home county, Cook County, with roughly 76 percent of the vote. He also swept the collar counties, with DuPage, Kendall, Lake and Will giving him double-digit margins. Notably, DuPage and McHenry hadn't supported a Democrat for president since 1948.[24] Obama also did very well in several rural counties that historically voted Republican. McCain did, however, win several of the more rural counties in Southern Illinois. It was not nearly enough, however, to put a serious dent in Obama's 25-point margin in the state.[25]

Technically the voters of Illinois cast their ballots for electors: representatives to the Electoral College. Illinois is allocated 21 electors because it has 19 congressional districts and 2 senators. All candidates who appear on the ballot or qualify to receive write-in votes must submit a list of 21 electors, who pledge to vote for their candidate and his or her running mate. Whoever wins the majority of votes in the state is awarded all 21 electoral votes. Their chosen electors then vote for President and Vice President. Although electors are pledged to their candidate and running mate, they are not obligated to vote for them.[26] An elector who votes for someone other than his or her candidate is known as a faithless elector.

The electors of each state and the District of Columbia met on December 15, 2008 to cast their votes for President and Vice President. The Electoral College itself never meets as one body. Instead the electors from each state and the District of Columbia met in their respective capitols.

The following were the members of the Electoral College from the state. All 21 were pledged to Barack Obama and Joe Biden:[27]